A friend wrote a blog post at http://www.huffingtonpost.co.uk/hannah-griffiths/isis-climat... whilst in Iraq, which refers to the Columbia University paper at http://www.pnas.org/content/112/11/3241.full

It's very much a picture of the drought causing people to move to overstretched towns and cities to look for work, and rising discontent with their government as a result. In some areas, mercenary work might be the only paid work available.